The present invention relates to a method and to a device for making a cylindrical metal structure referred to as body and consisting of several wires stapled to one another. This type of body is notably usable in the multilayer structures of reinforced flexible pipes for withstanding the stresses due to internal or external pressures or to longitudinal loads. One may refer to document API RP 17B (First edition, Jun. 1, 1988) which describes and defines flexible structures.
There are well-known  less than  less than rough bore  greater than  greater than  type pipes internally comprising a double-stapled metal section forming an internal body directly in contact with the fluid under pressure. The main purpose of this body is to withstand collapse under the effect of the external pressure transmitted by the extruded plastic sheath on this body. Other layers of internal pressure-resistant reinforcements, tension-resistant reinforcements, sealing plastic sheaths are arranged over this sheath. Document FR-2,654,795 describes a body made by plastic deformation of a flat metal strip such as a stainless steel strip in order to give it the shape of a double-stapled metal section. The body is formed by spiral winding of the strip, i.e. by small-pitch helical winding with interlocking, followed by a last deformation to provide stapling locking. It can be noted that, considering the manufacturing mode, the thickness of the strip cannot provide the final product with a very great transverse inertia, which limits the collapse strength of bodies according to the prior art.
The present invention allows to make a body with a greater collapse strength by using wires having a section of greater inertia. Wires of specific cross-section are therefore used to provide lateral stapling of the wires, i.e. limitation of the clearance between the turns in the direction of the longitudinal axis of the flexible pipe. In the present invention, the wires are not self-stapling wires, i.e. having a section with one and the same shape and arrangement, generally S or Z-shaped, which allows each turn to be fastened to the previous one by means of its complementary profiles.
Document FR-2,650,652, mentioned here by way of reference, describes wires having a T or U-shaped section. The U-shaped staplable (not self-stapling) wires can be described as having a substantially rectangular section comprising, at both ends of one side of the rectangle, generally the greatest length, convex ribs or bosses, thus forming the vertical stems of the U. When at least one of the turns is formed by a U-shaped wire, the helical winding is longitudinally locked by another U-shaped wire whose bosses are turned towards the bosses of the first wire and placed in the recesses of the U. There is thus no radial locking but only a longitudinal locking in relation to the axis of the pipe, with an allowable clearance according to the width and the space between the bosses. The absence of radial locking imposes relative precise control of the radial clearance between the inner wire and the outer wire because it is important, for the resistance to external pressure, that the wires are in contact or practically in contact with one another. The T-shaped staplable (not self-stapling) wires can be described as a U-shaped wire comprising, in the central part between the lateral bosses, a convex reinforcement (stem of the T) substantially perpendicular to the base (stroke of the T), the stem of the T of one of the wires being intercalated between the turns of the other wire.
The main difficulty for forming a body with staplable, and not self-stapling, wires (see above) having a great transverse inertia lies in the fact that the sections of the inner wire and of the outer wire do not have their axes of inertia, or their neutral fibers, equidistant from the axis of the pipe. In fact, the inner wire and the outer wire are arranged head to foot in relation to the stapling means, which leads to the fact that the axis of inertia of the outer wire is on a circle of greater radius than that of the axis of inertia of the inner wire, unlike self-stapling S or Z-shaped wires whose wire inertias coincide since the sections of the wires are identical and arranged similarly and at the same distance from the axis of the pipe. Thus, if preforming of the two inner and outer wires is performed identically with the same radius of curvature (on a supporting mandrel for example), relaxation of the deformation stresses, or elastic relaxation, can lead the outer wire to separate from the inner wire. This might lead to a decrease in the resistance to external pressure since the two inner and outer wires do not resist jointly. Furthermore, the body might come easily unhooked during later handling.
The present invention thus relates to a process for manufacturing a flexible continuous tube from two spiral wires whose T or U-shaped cross-sections comprise lateral bosses at the base of the U or the stem of the T, the two wires being arranged in relation to each other so that the bosses of one wire face the bosses of the other wire, stapling being performed by partial overlapping of one wire over the other. The process comprises the following stages:
injecting each wire into independent bending means consisting of at least three rollers arranged in relation to one another so that the wire, after bending, has the shape of a spiral of determined diameter and pitch,
adjusting the rate of injection of each wire according to the real length of each wire for the same number of turns,
arranging each bending means in relation to one another so that the turns of the wires are coaxial and that the wires, once bent, cross each other in order to be stapled by means of an elastic deformation in the radial and longitudinal direction.
In the process, the stapled position of the two wires can be secured by holding them one upon the other by means of two rollers.
The ratio of the areas of the sections of the inner and outer wires can range between 0.5 and 1.5.
The two wires can have identical T or U-shaped sections.
One wire can be T-shaped, the other U-shaped.
The invention also relates to an equipment for producing a flexible tube from two spiral wires whose T or U-shaped cross-sections comprise lateral bosses at the base of the U or the stem of the T, the two wires being arranged in relation to one another so that the bosses of one wire face the bosses of the other wire, stapling being performed by partial overlapping of one wire over the other. The equipment comprises bending means consisting of at least three rollers for each wire, bending means each connected to an independent frame and that can be positioned in relation to one another while maintaining the adjustment of the bending rollers, means for injecting the wire into said bending means and means for adjusting the injection rate.
The process is applied for producing a flexible tube body that must withstand an external pressure and comprising no inner tube.